It's hard to wrap my brain around it too, but some people act like jerks to hide their own insecurities. They'll call you girly because they wouldn't want to look like that and it would make them feel wrong, just like we feel in our born gender. It doesn't come from a place of concern for you, but the sentiment is the same. It's concern for themselves if they were you, with the exception of types who would get violent (unfortunately you can't easily tell), they'll be much nicer if you came out, ironically because now they can't put themselves in your shoes, because they'd never wear women's shoes :p.
It's the difference between being one of the guys, with something that's a bit different about you, and being one of the girls (maybe still with something a bit different about you, but that's ok.).
I overhear people talking about my butt, or my hair quite often, and sometimes they'll even say something like that to be mean, but in their head they're not being mean, they're just "busting your balls". People are never evil on purpose in my experience. Even the violent types think it's the right thing to do for some reason, at least in the moment. Most will feel ashamed after, hindsight is 20/20 even for them. I've been told I have a zen-like world view >.>